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Guidance on the Harmonized WAFS Grids for Cumulonimbus Cloud, Icing and Turbulence Forecasts

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Guidance on the Harmonized WAFS Grids for Cumulonimbus Cloud, Icing and Turbulence Forecasts Guidance on the Harmonized WAFS Grids for Cumulonimbus Cloud, Icing and Turbulence Forecasts. Version 2.5 13 September 2012 Guidance on the Harmonized WAFS Grids for Cumulonimbus Cloud, Icing and Turbulence forecasts. Contents 1. Introduction ....................................................................................................................... 2 2. General Description of Data ............................................................................................. 2 3. Description of Data Values ............................................................................................... 4 4. Grid Interpolation ............................................................................................................. 5 5. Description of the Harmonization Process ..................................................................... 5 6. Explanation of differences between the WAFS gridded forecasts for CB cloud, icing and turbulence and the WAFS significant weather (SIGWX) forecasts ............................. 6 7. Suggested usage of WAFS gridded forecasts for Cb cloud, icing and turbulence ...... 8 7.1 General principles ..................................................................................................... 8 7.2 Use of the WAFS Clear Air Turbulence (CAT) gridded forecast ........................ 8 7.3 Use of the WAFS icing gridded forecast ................................................................. 9 7.4 Use of the WAFS Cb cloud gridded forecasts ...................................................... 11 8. Quality control and continuous improvement .............................................................. 12 Appendix: Cumulonimbus Cloud, Icing and Turbulence Value Ranges and Missing Data Indicators ................................................................................................................................... 13 Version 2.5 13 September 2012 1 1. Introduction 1.1 The World Area Forecast System (WAFS) provides global gridded model forecasts of wind, temperature, relative humidity, turbulence, icing and cumulonimbus (Cb) cloud. The wind, temperature and relative humidity forecasts are available via SADIS 2G, SADIS FTP, Secure SADIS FTP and WIFS. The Cb cloud, icing and turbulence forecasts are available via SADIS FTP/Secure SADIS FTP and WIFS. Of these, the temperature and wind forecasts have been operational (through various GRID and GRIB formats) since 1983. Relative humidity was added in 2001. Cb cloud, icing and turbulence forecasts are currently provided for ‘trial and evaluation’ purposes, but have been recommended by the World Area Forecast System Operations Group (WAFSOPSG) to become operational with effect from November 2013 as part of Amendment 76 to ICAO Annex 3 – Meteorological Service for International Air Navigation, subject to successful verification of ‘harmonized’ versions of these forecasts and adoption by the ICAO Council. 1.2 This user's guide has been developed by the World Area Forecast Centre (WAFC) Provider States (the United Kingdom and the United States) to provide WAFS users with a general overview of the data, how it is calculated, and how it can be used. 2. General Description of Data 2.1 The data are made available in WMO GRIB2 format. Details on the format are available via the WAFC London WAFS Upper Air Forecast GRIB2 Dataset Guide, which can be found online. http://www.icao.int/safety/meteorology/WAFSOPSG/Pages/GuidanceMaterial.aspx 2.2 The target time for availability for wind, temperature and humidity forecasts is four hours and 55 minutes after the model data time. For example, the 1200 UTC model data delivery target time is 1655 UTC. 2.3 For the trial and evaluation forecasts of Cb cloud, icing and turbulence, the target time for the availability of the harmonized forecasts is 5 hours and 30 minutes after the model data time1. For example, the 1200 UTC model data delivery target time for these parameters is 1730 UTC. 2.4 For turbulence and icing, the value associated with a specific flight level (FL) should be considered applicable to the associated vertical layer. For example, a turbulence forecast at 300 hPa (FL300), would also be valid throughout the 50 hPa thick layer centered at 300 hPa, which extends from 325 hPa to 275 hPa. Table 1 below (and over) identifies the availability of the Cb cloud, icing and turbulence forecasts by flight level. A full list of all WAFS GRIB2 forecast parameters can be obtained via the WAFSOPSG web pages http://www.icao.int/safety/meteorology/WAFSOPSG/Guidance%20Material/WAFS%20GRIB 2%20Specification%20(V4.0).pdf . 1 Note, this exceeds the requirement in ICAO Annex 3 for delivery of WAFS gridded forecasts by 6 hours after model data time. Version 2.5 13 September 2012 2 Flight Pressure Mean/Max CAT Mean/Max in- Mean/Max Icing Height of Cb Level level (50hPa depth, cloud turbulence (100hPa depth, base/top, and (hPa) centered at level (100hPa depth, centred at level horizontal given) centred at level given) extent given) FL050 850 No No No, but base of N/A, existence lowest icing layer at a grid point starts at this level above the earth's FL060 800 No No yes, covering 850- surface will be 750hPa indicated by the FL100 700 No yes, covering 750- yes, covering 750- horizontal extent 650hPa 650hPa parameter, with FL140 600 No yes, covering 650- yes, covering 650- height of base 550hPa 550hPa and top FL180 500 No yes, covering 550- yes, covering 550- provided in the 450hPa 450hPa Cb base/top FL240 400 Yes, covering 425- yes, covering 450- yes, covering 450- fields. 375hPa 350hPa 350hPa FL270 350 Yes, covering 375- No, but top of No, but top of 325hPa lower layer and lower layer and base of higher layer base of higher layer meet at this level meet at this level FL300 300 Yes, covering 325- yes, covering 350- yes, covering 350- N/A, existence 275hPa 250hPa 250hPa at a grid point FL320 275 No, but top of No, but highest in- No, but highest above the earth's lower layer and cloud turbulence icing layer includes surface will be base of higher layer includes this this level indicated by the layer meet at this level horizontal extent level parameter, with FL340 250 Yes, covering 275- No, but this level is No, but this level is height of base 225hPa the top of the the top of the and top highest in-cloud highest icing layer provided in the turbulence layer Cb base/top FL360 225 No, but top of No No fields. lower layer and base of higher layer meet at this level FL390 200 Yes, covering 225- No No 175hPa FL450 150 Yes, covering 175- No No 125hPa FL530 100 No No No Table 1 Availability of the Cumulonimbus, Icing and Turbulence parameters by flight level Version 2.5 13 September 2012 3 3. Description of Data Values 3.1 Clear Air Turbulence (CAT) – is derived via an algorithm that is based on the Ellrod Index. The index is calculated from the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft. The resulting values provide an objective technique for forecasting clear-air-turbulence (CAT). The theoretical limit to the data range is zero to 99, but over 98 percent of the values will be below 11, and they will rarely exceed 40. The numbers are not a probability, but are instead a potential of encountering turbulence of any severity. 3.2 Since reports of CAT are subjective in nature, and depend upon the pilot's perception of the degree of turbulence, which in turn depends upon the weight, speed and aerodynamic characteristics of the aircraft being flown, it is difficult to establish thresholds to use when using the gridded data that are applicable to any aircraft type. 3.3 However, the limited range of values, combined with verification scores and comparisons to forecaster experience suggests that a value of 6 should be considered as a threshold for moderate or greater turbulence. It is recommended that users develop their own set of thresholds for their specific applications, much the same way that users have different thresholds for other weather variables, such as surface crosswind components for landings and takeoffs. 3.4 In-Cloud Turbulence – The in-cloud turbulence algorithms are based on 1) the model indicating the presence of a cloud, and 2) the change in potential energy with height, which is a measure of instability. There have not been any studies that suggest thresholds to use for these grids, however. The range of values in the data is from 0 to 1 and are a potential for encountering in-cloud turbulence. 3.5 Icing – The icing algorithms are based on a combination of cloud condensate (ice and water), temperature, relative humidity and vertical motion parameters that predict the presence of super-cooled liquid water. The values range from 0 to 1 and are a potential for the presence of icing. 3.6 They have not been calibrated to the probability of icing, but one study2 does suggest that a value of 0.1 should be used as a threshold for trace icing which could be used for Extended Diversion Time Operations (EDTO) (formally known as ETOPS) fuel use planning. This is because it provides a better determination of icing, with a lower false alarm rate than the alternative (traditional) method of using greater than 55 percent relative humidity with a temperature range from 0 to minus 20 degrees Celsius. 3.7 Cumulonimbus Clouds – the Cb cloud algorithm gives information relating to base, top and horizontal extent (coverage) of any expected Cb clouds. The horizontal extent component is expressed as a value between 0 and 1, representing the fraction
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